6505+ Has low output

[antknee75]

Just received a 6505+, via online purchase (Reverb).

It arrived looking great, but has very low output. With "Post" turned all the way up, it is still at what I would call practice or bedroom volume. At that level, it should be blowing walls down. This is a USA made version.

First thought is to replace power tubes, so I have some on order.
It appears to have the original unlabeled power tubes, and they all light up equally bright with no red plating or obvious signs of a problem. I attempted to remove the two outer and then the two inner tubes and to interchange them to see if it changed anything, but this had no effect on the issue.

Just wanted to see if there were any known quirks or common issues with this model that you more experienced users might know about.

Thanks, everyone!

[Enzo]

KNown quirks? It is just an amp like a thousand others. We just troubleshoot the problem.

I doubt the power tubes are at fault.

Plug a cord from FX send to FX return, any help?

Plug the guitar into the FX return, is that still low output? (None of the preamp controls will work this way)

And beyond those simple things, my first suspect is Q7, a muting JFET near the end of the preamp circuit.

[antknee75]

Enzo,
Thanks for the reply!

I tried installing new power tubes, and nothing changed.
Jumping the send to the return did not help.
Plugging straight into the return still has low output.

Thanks you again!

[Enzo]

OK, so issue is in power amp. Just a hunch, plug the speaker into the other speaker jack. I don't care if the 4 ohms or 8 ohms will be "wrong". it won't hurt it. See if it helps.

[antknee75]

OK, so issue is in power amp. Just a hunch, plug the speaker into the other speaker jack. I don't care if the 4 ohms or 8 ohms will be "wrong". it won't hurt it. See if it helps.

I've tried all sorts of cable and jack combinations and nothing changes.
I also pulled the chassis and checked for loose connectors or blown fuses. Everything looks fine.

I got sold a bad amp. If it's an easy fix I'll keep it, but I'm not going to bother with finding a tech and waiting for a repair. Right now, the turnaround time in my city is 4 to 5 weeks. I'd like to keep it, but I have initiated a claim and started return protocols with Reverb.

[antknee75]

Ok, so I settled with Reverb and I am keeping the amp.

These USA models are getting hard to find in good condition, so I negotiated a deal to keep it.

Now, if anyone wants to help me troubleshoot it, feel free to chime in!

Here's what we know:
The amp has very low output.
It is not the tubes.
Plugging straight into the effects return has low output.

[Enzo]

If the two speaker jacks work the same, then I would rule out the switching contacts in the one jack.

SO then apply a steady signal to the FX return and trace it through the return stage and into the phase inverter stage. Use either a scope or a "signal tracer".

[antknee75]

Ok. I'll start that process shortly.

I was just checking random components on the power board, and I have an open connection between the ends of CR200. I don't know what that means, but CR201 gets a voltage reading, CR200 does not.

Also found that the two blue wires for the power trans, on the power terminal strip, had come loose, so I resoldered them in place.

[antknee75]

Ok, I'm assuming I should be tracing the signal with the power off.

I seem to be losing the signal at relay K3. I can't get any signal at R33 or R85. (I don't know if power needs to be on for this.)

[antknee75]

Problem solved.

The signal passes through V4 on its way from the FX return to the power amp. I replaced V4 and I'm back in business!!!

Enzo, thanks for your attention. I really appreciate it!

[Enzo]

Great!!!


But for future reference, a signal will not get far through a circuit with power off. We trace a signal in an active circuit.